For thirty years or more, aluminum scrap such as shredded mixed "used beverage containers" (UBCs) has been recycled as a cost effective way to manufacture aluminum metal, as contrasted with the refining of aluminum ore. U.S. patents representative of basic recycling technology include, for example, U.S. Pat. Nos. 3,997,336, No. 4,128,415, and No. 4,983,216, all to van Linden et al. When shredded mixed UBCs were historically recycled without removal of the coatings thereon, the aluminum recovery during recycling was very poor. An early process for coating removal, which is still in use at this writing, is a thermal delacquering process in which shredded mixed UBCs are subjected to heating to a temperature slightly under the melting point of the aluminum. The high temperatures literally incinerate the coatings so as to remove them, and delacquered shredded mixed UBCs of this type can be recycled with very high recovery of aluminum metal.
Thermal delacquering has at least two major disadvantages, however, as a method for removing coatings from aluminum scrap prior to recycling. First, the gaseous effluent created from what amounts to combustion of the coatings requires further environmental remediation. Also, thermal delacquering of aluminum scrap itself creates a loss of some of the otherwise recyclable metal.
Other prior art efforts to remove coatings have included the use of chemical delacquering compositions. For example, Fujisawa et al., "Swell-Peeling Method for Paints on Aluminum Cans," Metallurgical Review of Mining and Materials Processing Institute, vol. 16, no. 1, 1999, discuss the use of methylene chloride alone or in combination with formic acid and/or acetic acid as a composition to promote the swelling, peeling and removal of paints coated on aluminum cans. The combination of methylene chloride, formic acid and halogenated acetic acid is reported to effect better coating removal than can be achieved with methylene chloride alone or methylene chloride in combination with formic acid. Unfortunately, the compositions of Fujisawa et al. and others are plagued by environmental unsuitability. Methylene chloride and formic acid are each both volatile and hazardous, which make handling these materials cumbersome and expensive. Prior art attempts to use methyl ethyl ketone or sulfuric acid created similar environmental obstacles and, ironically, many strong acids such as sulfuric acid proved surprisingly ineffective as delacquering agents.
Certain prior art U.S. patents have addressed the chemical removal of coatings from aluminum or metal scrap. U.S. Pat. No. 5,817,163 to Wood describes a two step process which applies superheated steam @ 1000.degree. F. to the coated scrap prior to traditional thermal delacquering. The superheated steam is disclosed as producing a carbon-containing volatile from the carbon-containing coatings, which carbon-containing volatile can be removed from the spent steam to prevent its fugitive emission. U.S. Pat. Nos. 5,423,922 and No. 5,494,603 to Kirk disclose a delacquering composition containing primarily polyalkylene glycol polymer and an aqueous solution of a smaller amount of a dicarboxylic acid such as oxalic, malonic, succinic, glutaric, or adipic acid (alone or in combination). The disadvantage with superheated steam is its higher energy requirement and inconsistent effectiveness. Polyalkylene glycol polymers are difficult to handle, inasmuch as they require consideration of human exposure issues and waste disposal issues, and would also inevitably partially remain on the metal thus treated. During subsequent heating and melting, any remaining polyalkylene glycol polymer would then likely decompose and release undesirable emission products, and would likely also contribute to unwanted metal oxidation at the same time.
In view of the disadvantages by which the prior art is plagued, the present invention seeks to provide delacquering of coatings from coated aluminum and other scrap or substrates by means of compositions and methods which give improved metal recovery yields and which are environmentally acceptable or optimal.